Secondary or rechargeable batteries are used in a variety of applications, but the secondary battery market is coming under increased attack by environmental concerns. Indeed, the principal consumer rechargeable battery, the nickel-cadmium battery, has limited future utility because of growing governmental regulatory pressures. Current alkali metal batteries have not been used extensively as secondary batteries because of their limited cycle life.
Conventional secondary, non-aqueous lithium cells typically include an anode of metallic lithium, a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents, and a cathode of an electrochemically active material, typically a chalcogenide or oxide of a transition metal. During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode where the ions are taken up with the simultaneous release of electrical energy. During charging, however, the flow of ions is reversed so that lithium ions pass from the electrochemically active cathode material through the electrolyte and are plated back onto the lithium anode. During each discharge/charge cycle small amounts of lithium and electrolyte are consumed by chemical reactions at newly created surfaces. As lithium inherently tends to form high surface area peaks or dendrites as it is plated back onto the anode, this reactive condition is aggravated. Furthermore, the dendritic peaks continue to grow until they eventually contact the cathode which causes the cell to fail before the useful lifetime is realized.
In an attempt to solve these problems, carbonaceous material as lithium intercalation anodes in secondary lithium-ion batteries have been introduced. Yoshiro et al., U.S. Pat. No. 4,668,595, issued May 26, 1987; Basu, U.S. Pat. No. 4,423,125, issued Dec. 27, 1983; and Basu, U.S. Pat. No. 4,304,825, issued Dec. 8, 1981. Carbon anodes have been synthesized from various organic compounds by vapor phase pyrolysis. In this process molecules are vaporized in flowing argon or nitrogen and then pyrolyzed onto an anode substrate. Benzene is a preferred organic precursor and some benzene derived anodes have exhibited up to approximately 86% lithium utilization efficiency (Li.sub.0.86 C.sub.6) through approximately 100 cycles. Mohri et al., U.S. Pat. No. 4,863,814, issued Sep. 5, 1989; Yoshimoto et al., U.S. Pat. No. 4,863,818, issued Sep. 5, 1989; and Yoshimoto et al., U.S. Pat. No. 4,968,527, issued Nov. 6, 1990. Carbonaceous anodes can also be formed by condensed phase pyrolysis of individual organic compounds, including polyacrylonitrile (PAN). Hiratsuka et al., U.S. Pat. No. 4,702,977, issued Oct. 27, 1987. Alternatively, the carbonaceous materials can be blended with powdered metals that alloy with lithium. Miyabayashi et al., U.S. Pat. No. 4,945,014, issued Jul. 31, 1990. See also Murakami et al., U.S. Pat. No. 4,749,514, issued Jun. 7, 1988 (pyrolysis of thin films of poly(phenylene oxadiazole) and Nishi et al., U.S. Pat. No. 4,959,281, issued Sep. 25, 1990 (carbon anodes produced by pyrolysis of selected furan resins). Anodes having a laminate structure consisting of a carbon molded article, made of carbon fiber or carbon powder, with lithium absorbed therein before being inserted into the secondary battery have also been used. Takahashi et al., U.S. Pat. No. 4,980,250, issued Dec. 25, 1990. Finally, carbonaceous anodes consisting of a multi-phase composition that includes (1) a highly graphitic carbon, (2) a substantially non-graphitic carbon, and (3) an electrically conductive filamentary material, such as carbon black, have been introduced. Fong, U.S. Pat. No. 5,028,500, issued Jul. 2, 1991.
Some of these prior art methods which endeavored to control electrode porosity did not produce quality electrodes because they invariably used binders, adhesives, or thermal plastics to convert carbon powders into porous sheets or plates. The use of these materials frequently actually degrades the utilization efficiency of the carbon electrodes and reduces the electrical conductivity of the electrodes, the latter phenomenon being caused by the particle-to-particle resistive contacts in such bonded composites.
Thus, despite some improvements in alkali secondary batteries, there remains a need for rechargeable batteries that are inexpensive, stable, easy to manufacture, and that have extended rechargeable lifetimes.